1. Technical Field
The present invention relates generally to vehicle power trains and more particularly to a transmission assembly which utilizes an improved method for controlling the engagement of a torque converter clutch.
2. Discussion
Modern automotive vehicles typically include a power plant, such as an internal combustion engine, a power train, and wheels. The power plant converts fuel into rotational energy having torque and speed characteristics. The power train is operable for transmitting the rotational energy from the power plant to the wheels to propel the vehicle.
The main component of the power train is generally referred to as a "transmission assembly". The transmission assembly is operable for converting the speed and torque produced by the power plant in accordance with the tractive-power demand of the vehicle. A conventional transmission assembly typically includes a hydrodynamic torque converter and a gearbox. The torque converter transfers engine torque from the engine crankshaft to a rotatable input member of the gearbox through fluid-flow forces.
Generally speaking, the torque converter includes a fluid, an impeller which is coupled for rotation with the engine crankshaft, a turbine coupled for rotation with the gearbox input shaft, a stator for redirecting fluid flow from the turbine side to the impeller to effectively multiply engine torque, and a converter clutch operable for "locking" the impeller and turbine such that they rotate at the same speed (i.e., engine crankshaft speed). A thorough description of general automatic transmission design principals may be found in "Fundamentals of Automatic Transmissions and Transaxles," Chrysler Corporation Training Manual No. TM-508A. An additional description of torque converters may be found in U.S. Pat. No. 4,289,048, entitled "Lock-up System for Torque Converter," issued on Sep. 15, 1981 to Mikel, et al. which is hereby incorporated by reference.
Modernly, vehicle power trains have been equipped with electronically controlled converter clutches in an effort to optimize the efficiency and performance of the power train, as well as reduce the operating temperature of the fluid in the torque converter. In the absence of extraordinary operating conditions, the prior art electronic converter clutch control (ECCC) methods are typically used to engage the converter clutch when a vehicle transmission is engaged in its highest gear ratio (e.g., "fourth gear" in a transmission having four gear ratios). The use of the prior art ECCC methods in the lower gear ratios, however, has produced undesirable "driveability" issues. These issues stem from the typically transient use of the lower gear ratios and the fact that vehicles are seldom operated while these gear ratios are employed. Compounding matters is the fact that the power train is typically operating at relatively light loads while these gear ratios are in use so that the transmission fluid is typically exposed to low to moderate levels of heat.
However, the load on the power train substantially increases where the vehicle is operated up a steep incline or is used to tow an object. Despite the relative rarity of these situations, they can significantly increase the temperature of the transmission fluid and as such, the vehicle must be equipped to sufficiently reject this heat. This has led to the use of oversized cooling systems and optional boost coolers to provide sufficient cooling capacity to reject the additional heat generated when operating a vehicle under these conditions. This solution, however, is relatively costly.
Therefore, there remains a need in the art for an improved ECCC method that may be employed with lower gear ratios to reduce the amount of heat produced during periods where the power train is heavily loaded and which does not impact the driveability of the vehicle. Employment of an improved ECCC method while the lower gear ratios of a gearbox were engaged would also improve the fuel economy of the vehicle as the inefficiencies associated with the slip between the impeller and the turbine would be eliminated.